Eddy-current effects tend to force high frequency RF currents towards the outer surface of any conductor, biological or metal. This tendency, known as the “skin effect,” is dependent upon the bulk resistivity of the conductor and the operating frequency.
At the electrode-patient contact in electro-surgical applications, the skin effect tends to force currents towards the edge of the electrode, resulting in significant tissue heating at the electrode edges. This is a major concern in electro-surgical treatments since second or third degree burns are possible, particularly if the patient is anesthetized.
Adding a distributed reactance to the electrode contact surface significantly reduces burn risks by cancelling the skin effect to first order, producing a considerably more uniform heating profile. However, since bulk resistivity is a direct factor in the skin effect equation, changes in the tissue resistivity surrounding the electrode can still significantly alter the heating profile during treatment if the operating frequency is fixed.
There are several known approaches to addressing this problem. In one such approach, a fixed operating frequency is selected from multi-dimensional lookup tables, based upon measurements of fat thickness and other empirical parameters. In a second approach, treatment is performed at a fixed power level or power cycling profile and is terminated upon indication of excessive skin temperature. In a third approach, treatment is performed at a fixed power level or power cycling profile and is terminated upon patient request.
It is, however, desirable to have available a treatment system and method that eliminates the need for lookup tables and actively maintains an optimal spatial heating profile under varying load resistivity, thereby reducing the risk of burns and maximizing patient comfort at a given power level.